Re: [PATCH 0/6] Support for LEGO MINDSTORMS EV3 LCD display

From: Noralf TrÃnnes
Date: Tue Aug 01 2017 - 14:09:19 EST

(cc: Daniel Vetter)


Den 01.08.2017 18.51, skrev David Lechner:
On 07/30/2017 12:14 PM, Noralf TrÃnnes wrote:

Den 29.07.2017 21.40, skrev David Lechner:
On 07/29/2017 02:17 PM, David Lechner wrote:
The goal of this series is to get the built-in LCD of the LEGO MINDSTORMS EV3
working. But, most of the content here is building up the infrastructure to do
that.


Some general comments/questions:

I have noticed that DRM doesn't really have support for monochrome displays. I'm guessing that is because no one really uses them anymore?


The repaper driver was the first monochrome drm driver and I chose to
present it to userspace as XRGB8888 and convert it to monochrome.
The reason for this is that everything, libraries, apps, plymouth (boot
splash, no rgb565) supports it. I didn't see any point in adding a new
monochrome drm format that didn't have, or probably wouldn't get
userspace support (by libraries at least). The application of course
needs to know this to get a good result.

tinydrm_xrgb8888_to_gray8() does the conversion:
https://cgit.freedesktop.org/drm/drm-misc/commit/drivers/gpu/drm/tinydrm?id=379ea9a1a59a5a32c8db6f164e80a3fd00cb3781

The LEGO EV3 display is just an LCD (not the backlit kind). It has two modes of operation. It can to 2bbp grayscale or it can do 1bpp monochrome. The grayscale isn't the best (looks splotchy in places), so it would be nice to be able to choose between these two modes. How would I implement something like that?


Do you expect anyone to use grayscale if it doesn't look good?
Maybe better to add it later if there's a demand for it.

I don't expect many people to use it at all. The people that do will be hackers like me that want to see what it is capable of, so I think I will keep it grayscale by default.


It looks like the best way to satisfy your needs is to add specific formats.

Video for Linux have these:

include/uapi/linux/videodev2.h

/* Grey formats */
#define V4L2_PIX_FMT_GREY v4l2_fourcc('G', 'R', 'E', 'Y') /* 8 Greyscale */
#define V4L2_PIX_FMT_Y4 v4l2_fourcc('Y', '0', '4', ' ') /* 4 Greyscale */
#define V4L2_PIX_FMT_Y6 v4l2_fourcc('Y', '0', '6', ' ') /* 6 Greyscale */
#define V4L2_PIX_FMT_Y10 v4l2_fourcc('Y', '1', '0', ' ') /* 10 Greyscale */
#define V4L2_PIX_FMT_Y12 v4l2_fourcc('Y', '1', '2', ' ') /* 12 Greyscale */
#define V4L2_PIX_FMT_Y16 v4l2_fourcc('Y', '1', '6', ' ') /* 16 Greyscale */
#define V4L2_PIX_FMT_Y16_BE v4l2_fourcc_be('Y', '1', '6', ' ') /* 16 Greyscale BE */


Maybe we can add formats like these:

include/uapi/drm/drm_fourcc.h

#define DRM_FORMAT_MONO fourcc_code('Y', '0', '1', ' ') /* [0] Monochrome */
or
#define DRM_FORMAT_Y1 fourcc_code('Y', '0', '1', ' ') /* [0] Monochrome */

#define DRM_FORMAT_Y2 fourcc_code('Y', '0', '2', ' ') /* [1:0] Greyscale */

Daniel, what do you think?


Also, how can I indicate to userspace that this display really is monochrome/grayscale since the reported color depth 16bpp?


There isn't unless we add formats for it.
Since this display is in a Lego piece, doesn't it have custom userspace
that already know it's monochrome?

The official LEGO software is like this, yes. But I am working on a project that supports other LEGO compatible devices that have color screens, so the same software needs to be able to detect at runtime which type of screen it is using. Currently I have a plain fbdev driver that provides FB_VISUAL_MONO10 for the EV3 display and so the software knows when it has a monochrome screen and when it doesn't. But since plain fbdev drivers can't be accepted into mainline, I need to find a different way to detect what type of screen this is so that my graphics library can treat it differently.



You can tell userspace about the preferred bitdepth:
drm->mode_config.preferred_depth


Noralf.